Fix rich-inline punctuation boundary wrapping

[luyh7]

Summary

• Keep rich-inline item boundaries from wrapping before line-start-prohibited punctuation when there is no collapsed whitespace gap.
• Reuse the core punctuation sets so split inline items such as Hello + . stay attached.
• Add a regression test covering materialized rich-inline lines and stats.

Why

Adjacent inline items should not introduce an extra line break before punctuation that browsers keep attached to the previous text node. This addresses #177.

Validation

• /home/luyh7/tmp/.tools/bun-1.3.14/package/bin/bun test src/layout.test.ts
• node node_modules/typescript/bin/tsc --noEmit
• node node_modules/typescript/bin/tsc -p tsconfig.build.json
• git diff --check

[rf-]

If it's true that browsers behave exactly the same way for adjacent spans that they do for a single text node, it seems like doing this correctly would require a much bigger refactor so that all of the logic in buildMergedSegmentation can be applied across items. That's pretty difficult though—I'm taking a stab at it but haven't fully proven it out yet, and it's hard to not regress on performance at all in the simple case.